Determining the thickness of materials grown or deposited onto a substrate is crucial to the performance of an end product in various areas such as, but not limited to: microelectronics, such as semiconductor and integrated circuit (IC) fabrication; coatings, such as thermal barrier coatings; precision optics; etc. For example, the thickness of thin films grown or deposited onto a substrate controls the resistivity and heat generated, among other variables, in a design. Thicknesses that are too great or too little can adversely affect the performance of a device, thereby affecting the yield of a process run. Film uniformity also affects the performance of a film, and accordingly the performance of a device incorporating a film.
Thickness measurement of films, for example, can generally be grouped into two categories: contact and non-contact. Contact methods include atomic force microscopy (AFM) and use of a profilometer. However, these contact methods typically cannot be used for in-situ film inspection.
Non-contact methods include gravimetric, eddy current, and optical methods. A quartz crystal monitor (QCM) gravimetric method is widely identified for in-situ characterization of thin film thicknesses. QCM measures film thicknesses by monitoring the frequency shift of a quartz crystal as material is deposited on it. Though this method is in-situ, it is also remote. That is, film thickness monitoring does not actually happen on the sample on which the film is being deposited.
Use of eddy currents to detect film thicknesses is useful for measuring metal film thicknesses, but requires large scanning times that are not suitable for in-situ measurements. Also, some in-situ methods of deposition create electromagnetic fields that may affect eddy current measurements.
Optical methods can be implemented to monitor film growth/deposition and measure film thicknesses. Such methods are designed such that electromagnetic fields do not interfere with them, and they can be done in real time. Optical methods, therefore, are the only methods that can be implemented into a real-time production environment to improve the production yield while increasing the production rate.
Most optical methods rely on the amount of light reflected and/or transmitted by the substrate. Ellipsometry uses polarized light to determine film thickness and other film parameters, but is limited to point measurements due to the necessary polarizers and other adjustments that need to be made to obtain measurements. This makes ellipsometry unsuitable for real-time full-field measurements. While several point measurements can be made by ellipsometry to take thickness maps, this takes a significant amount of time and is not suitable for in-situ measuring. Other methods that do not use polarized light make use of the interference of light, but are confined to point measurements or very small areas. Point and small area measurements also provide little to no information about the uniformity of a film.